Override final pixel color in a Cycles/OSL shader?
I do believe that RSL scripts might have more control over the final colour than OSL scripts do. From the OSL specification --


  A closure is an expression or function call that will be stored, along with necessary contextual information, to be evaluated at a later time.


What this means is the green node sockets that connect to the Material Output provide the render engine with a way to calculate the final colour not the specific colour to use in the final image. The steps of mixing reflections and lighting is performed by the renderer not by the OSL script.

For interest the first example that gandalf3 pointed you to can be achieved in OSL with --

shader flatcolour(color Colour = 0.8,
    float Strength = 1.0,
    output closure color BSDF = 0 )
{
    if(raytype("camera"))
        BSDF = (Strength * Colour) * emission();
    else
        BSDF = transparent();
}